Don't overload FreeListElement for become.

runtime/vm/object_test.cc

Index: runtime/vm/object_test.cc
diff --git a/runtime/vm/object_test.cc b/runtime/vm/object_test.cc
index 9ad5f70d3069cebb8e6af3b3862ba65b8e7af959..7d70f200231603b73b309a9232c917a30e00f560 100644
--- a/runtime/vm/object_test.cc
+++ b/runtime/vm/object_test.cc
@@ -4125,9 +4125,8 @@ class ObjectAccumulator : public ObjectVisitor {
       : objects_(objects) { }
   virtual ~ObjectAccumulator() { }
   virtual void VisitObject(RawObject* obj) {
-    // Free-list elements cannot even be wrapped in handles.
-    if (obj->IsFreeListElement()) {
-      return;
+    if (obj->IsPseudoObject()) {
+      return;  // Cannot be wrapped in handles.
     }
     Object& handle = Object::Handle(obj);
     // Skip some common simple objects to run in reasonable time.